Global Warming - Energy and the Environment - page 294

(mostly tropical, less-developed countries) to plant new trees. The emitting countries would obtain
credit for CO 2 emission reductions equivalent to the amount of CO 2 absorbed by the new trees.
This is called emission trading. The exact accounting procedures for such emission trading have
not yet been established.
Finally, one must note that deforestation has the opposite effect. Every hectare cleared of
forests will reduce the absorption of CO 2 by6-10tCha − 1 y − 1 . And if the trees are burned, as
it is still widely practiced in tropical countries of the world, there is a double effect: cessation of
absorption of CO 2 and emission of CO 2 due to wood burning. It is estimated that forest burning
alone adds about 1.5 Gt y − 1 of carbon to the atmosphere.

10.4.6 Shift to Non-fossil Energy Sources

It goes almost without saying that the CO 2 -caused climate warming can be ameliorated by shifting
to nonfossil energy sources. In the previous section we have seen that a shift to biomass would be at
least CO 2 neutral; that is, as much CO 2 being emitted into the atmosphere from biomass use will be
reabsorbed in future growth of trees and plants that produce the biomass. Shift to noncarbon energy
sources eliminates CO 2 emissions completely, except perhaps for the fossil energy (mainly coal)
used in smelting steel and other construction materials used in the nonfossil energy conversion
devices.
In Section 2.3 we have seen that worldwide energy consumption is supplied by the following
sources: 86% fossil, 6.5% nuclear, and 7.5% renewables, with the latter almost entirely hydroen-
ergy. Because a shift to nuclear energy appears for the foreseeable future unacceptable to the public
and body politic in most countries, the only recourse would be a shift to renewables.
Among the renewables, hydroenergy also appears to run into public opposition. 17 Thus, all
expectations are turned toward solar, wind, geothermal, and ocean energy. These technologies
are described in detail in Chapter 7. Their substitution for fossil energy is mostly dependent on
economics. Their slice of the energy pie will only increase if the cost of renewable energy conversion
devices becomes cheaper, or the price of fossil energy becomes greater.

10.5 CONCLUSION

Global warming is caused by increasing concentrations of far-infrared absorbing gases, called
greenhouse gases (GHG), including CO 2 ,CH 4 , CFCs, N 2 O, and O 3 . Most of these gases are of
anthropogenic origin, primarily associated with fossil fuel usage. Water vapor is also a greenhouse
gas, but of course it is of natural origin, although the global hydrogeologic cycle may be altered to
some degree by anthropogenic activities.
The presence of GHG in the lower atmosphere causes the outgoing terrestrial far-infrared
radiation to be absorbed and partially reradiated toward the earth's surface. This radiation increases
the earth's surface temperature. There is substantial evidence that the average earth's surface
temperature already has increased by about 0.5-1.0 ◦ C since the middle of the nineteenth century,
and by the end of the twenty-first century the surface temperature may rise by another 2-3 ◦ C. This

17 Note, for example, the national and international criticism that is aimed at the government of China for the
construction of the 18-GW hydroelectric project on the Yangtze River at the Three Gorges Dam.